Devices of the class to which the present invention relates are commonly called "off-delay" or "delay-on-break" relays or timers. They are so called because they provide a time delay function which commences upon de-energizing or "breaking" an electrical or electronic circuit. A typical delay-on-break relay includes a switching device such as silicon controlled rectifier, triac or electromechanical relay which can be connected in series with a load. The switching device is controlled by a timer to prevent the load from being reenergized until at least a predetermined period of time elapses after the most recent prior energization of the load. Various circuits for implementing a delay-on-break function are shown for example in U.S. Pat. Nos. 3,814,991 to Hewitt; 3,619,668 to Pinckaers; 3,636,369 to Harter; 3,742,303 to Dageford; 3,864,611 to Chang; 4,281,358 to Plouffe; 3,946,574 to Portera and 4,991,049 to Kadah.
Delay-on-break devices are used in a wide variety of control applications as well as to protect certain types of loads from damage which might otherwise be caused by cycling them off and on too rapidly. One common application of delay-on-break devices is the protection of compressor drive motors in air conditioning and refrigeration equipment. Such motors are often capable of delivering only modest starting torque even though they are capable of driving much greater loads at typical operating speeds. If an air conditioning or refrigeration system has been off for a sufficient length of time, the pressure across the compressor, and the corresponding starting load on the motor, are low enough to permit the motor to start and drive the compressor even with the modest starting torque available. As the motor picks up speed, the compressor raises the pressure in the system and the load on the motor increases substantially. When the motor is deenergized, the pressure in the system decreases relatively gradually over time. If the motor is reenergized prior to the time the pressure drops to a level within the starting torque capabilities of the motor, the motor will stall. In the stalled condition, heat rapidly builds up within the motor to a point capable of damaging the motor or even causing a fire. A delay-on-break device can prevent this from occurring by insuring that the motor remains off for a period of time sufficient to allow the pressure to drop to a point at which the motor is capable of starting reliably. In other applications, in which low starting torque may not be a problem, delay-on-break devices can be used to protect motors, solenoids or other inductive loads subject to high inrush currents from excessive heating in the event they are repetitively energized and deenergized at too rapid a rate.
For A.C. applications, triacs are often preferred for use as the switching device in delay-on-break timers. Using the triac gate as a control input, a small current applied to the gate can be used to selectively enable the flow of a much larger current through the load. Triacs are also capable of conducting currents of substantial magnitude over both the positive and negative half cycles of the A.C. waveform with only slight voltage drop. Triacs suffer from a limitation however in that they normally cease conducting when zero crossings of the A.C. waveform are encountered unless the gate is retriggered.
To overcome this inherent limitation of triac operation in A.C. circuits, it has long been known to those of ordinary skill in the art to connect a latching capacitor of sufficient capacitance between the gate of the triac and one of the conductors of the A.C. line and to connect the load between that conductor and the first main terminal of the triac. Conduction of the triac through zero crossings of the A.C. waveform is maintained, i.e., the triac is "latched" by the capacitor as the capacitor discharges at least partially to supply a gate current sufficient to retrigger conduction of the triac notwithstanding the zero crossings of the voltage waveform on the A.C. line. This latching capacitor technique has long been taught in standard manuals describing how to use triacs and other thyristor devices such as at page 199 of the SCR Manual Including Triacs and Other Thyristors, Sixth Edition, copyrighted 1979 by the General Electric Company.
U.S. Pat. No. 4,991,049 to Kadah shows the use of a latching capacitor in a protective time delay circuit for delaying energization of an electric load until a predetermined time has elapsed from a prior deenergization. Kadah '049 shows a PNP transistor whose emitter is coupled to one side of the A.C. line through a rectifier in series with a resistor. Where it is desired to protect the load from possible damage due to low voltage (i.e., "brownout") conditions, a zener diode can be added in series with the emitter. The base of the transistor is coupled to the other side of the A.C. line by way of an RC timing network in the form of a resistor in parallel with a timing capacitor. The collector of the transistor is connected to the gate of a triac whose main terminals are in series with a load such as the coil of a compressor motor relay. A latching capacitor is connected between the gate of the triac and the side of the A.C. line common to one side of both the R.C. network and the load.
When the A.C. line is first energized, the timing capacitor is initially discharged and the transistor is biased into conduction allowing sufficient gate current to flow to turn on the triac and energize the load. The load remains energized through A.C. zero crossings due to the action of the latching capacitor. When the timing capacitor charges sufficiently, the transistor is biased off except that some current from the triac gate can flow through the collector-base junction, when it is forward biased, in order to maintain the charge on the timing capacitor while the A.C. line remains energized. Upon deenergization of the A.C. line, the triac ceases conducting and cannot be turned on again even if the A.C. line is reenergized until the timing capacitor discharges sufficiently to permit the transistor to be biased into conduction. Thus the transistor serves two roles. It controls gating of the triac according to the bias established by the timing capacitor and it also maintains the charge on the timing capacitor at all times while the load is energized.
One limitation of prior art protective devices of the above type is that their timing performance requires close attention to variations in tolerances among various discrete devices including significantly, transistor parameters. They also require the use of a capacitor for latching and to provide a brownout protection, a separate non-capacitive element, such as a zener diode connected in series with the emitter of the transistor. While such an arrangement can prevent the load from being switched from a de-energized state to an energized state if the line voltage is too low, it is of limited protective value in that it will not cause the load to be rapidly de-energized if a brownout occurs when the load is already energized.